The present invention relates to apparatus and methods for purifying flue gases. The present invention relates specifically to a nozzle apparatus in a gas conduit or in an activating reactor for spraying a wetting medium, such as water, water vapor or other wetting liquid into the gas flow. The nozzle apparatus comprises a nozzle body arranged in the gas conduit or activating reactor, the nozzle body being provided with at least one, preferably several, nozzles for spraying the wetting medium into the gas flow.
More specifically, the present invention relates to a nozzle apparatus suitable for use with different gas purification methods in connection with combustion, gasification or chemical or metallurgical processes. Sulphur oxides, ammonium, chlorine and fluorine compounds and condensable hydrocarbon compounds are typical impurities of gases formed in these processes. The gases are purified, e.g., by adding an absorbent and/or reagent reactive with the impurities into the gases during either the actual process or during the purifying stage subsequent to the process. The reagent or absorbent is activated during the purifying stage by leading the gases through an activating reactor, in which both the gases containing impurities and the reagents and/or absorbents are wetted with water or water vapor sprays from the nozzle apparatuses. Water or water vapor sprays form a wetting zone in the reactor, and the reagents or absorbents are activated in the wetting zone. After the purifying stage the completely or partly reacted reagent and/or absorbent particles are separated from the gases. Alkali metal or alkali earth metal carbonates, oxides or hydroxides, for example, are used as reagents and/or absorbents.
It is well known, that combustion of fossil fuels forms flue gases containing sulphur oxides, which in turn cause acidifying of the environment. The sulphur content of the flue gases varies depending on the sulphur content of the fuel. Methods to utilize fuels of greater sulphur content are being examined, even though the sulphur emission restrictions are being tightened. Waste incineration plants, the number of which is increasing, also form sulphur containing flue gases that will have to be purified so as to meet the acceptable emission levels. The flue gases from waste incineration plants contain, besides SO.sub.2 - and SO.sub.3 - emissions also, e.g. when combusting plastic compounds, hydrochloride and hydrofluoride acids and other harmful gaseous and solid compounds.
The process gases formed in different gasification processes can also contain harmful amounts of sulphuric or other compounds that have to be removed from the gases prior to further treatment of the gases.
Several methods have been developed for reducing sulphur emissions from combustion plants. A recent development in the flue gas purifying field has been the so-called semi-dry scrubbing method, in which a fine alkali suspension, e.g. a calcium hydroxide suspension, is sprayed through nozzle apparatuses into a hot flue gas flow in a contact reactor where the sulphur dioxides dissolve in water, and as the suspension dries, are bound to the lime compound. The water is evaporated in a contact reactor so that a solid residue is formed, whereby the reaction products of e.g. sulphur and lime are easily separable from the gases with a filter. There is a tendency to try to maintain the calcium hydroxide suspension at such a concentration that the water can be evaporated therefrom by the heat of the flue gases. A concentrated lime suspension, nevertheless, easily forms layers on the reactor walls and specially in the vicinity of the injection nozzles, which can finally be clogged.
The semi-dry method is preferable for the process because the impurities can be removed from the gases as solid waste, the disadvantage of the method nevertheless being the difficulty of controlling the process.
It has been previously suggested that the limestone is added straight into the actual combustion or gasification stage, whereby the limestone is calcined into calcium oxide according to the following reaction: EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2
Calcium oxide can react already in the combustion stage with the sulphur dioxides formed therein as described by the formula
CaO+SO.sub.2 +1/202.fwdarw.CaSO.sub.4.
But as the reactions advance, the calcium oxide particles are covered by layers of calcium sulphate or calcium sulphite which impede the sulphur from penetrating into the particles and thereby slow down and finally prevent all reactions between sulphur and lime. Thus lime does not react completely and is thereby not optimally utilized. Other factors, such as Ca/S mole ratio, temperature and retention time do also have an effect on sulphur absorption. It is, however, known that the reactivity of alkali compounds increases in relation to the proximity of dew point. The increased reactivity is due to the fact that in a wet particle the reactions take place in water phase as fast ion reactions. Close to the dew point the particles remain wet and therefore also their reactivity remains good for a longer period. Therefore, it has been suggested that the gases should be led through a wetting reactor, wherein water or water vapor sprays maintain the particles preferably wet enough for the water to surround the particles and also to penetrate into them. As the water penetrates into the lime particles, the sulphate or sulphite layer formed around the particles breaks and thus new reactive lime surface is exposed. The sulphur dioxide dissolves from the gases into the water and reacts with the calcium compounds in liquid phase on the surface of the particles.
Finnish patent application FI 900915 discloses a wetting reactor in which the particles are dry when discharged therefrom, even though the gas flow is wetted to very close to the dew point, even as close as 0.degree.-20.degree. C. from the dew point. According to the method the gases to be purified are introduced into the wetting reactor into at least two levels. In the wetting reactor the gases flow upwards and are exhausted from the upper portion of the wetting reactor. The gases are introduced into the wetting reactor so that a first portion of the gases is introduced into the wetting zone, in which the gas suspension formed of gas and reagent and/or absorbent is wetted with water and/or water vapor. A second portion of the gases is introduced into a drying zone below the wetting zone. In the wetting zone, the particle density of the particle suspension is maintained higher than that of the gas introduced into the wetting reactor by recycling particles separated by e.g. a filter in the upper portion of the wetting reactor back to the wetting zone. Thus, an internal recycling of the reagent or absorbent particles is effected in the wetting reactor and the particle density is maintained at a relatively high level.
Water or water vapor sprays thus generate a wetting zone in the upper or central portion of the wetting reactor. Water is sprayed into the flue gases mainly from above the gas inlets by preferably downward directed water or water vapor nozzles, arranged on support members spanning the wetting reactor horizontally. The support members can act as wetting medium conduits, through which the water or water vapor is conducted to the nozzles. Usually the water or water vapor sprays are directed so as to evenly cover as large a part of the gas flow as possible.
The gas introduced below the wetting zone rises and thereby acts advantageously as so-called drying gas and is brought into contact with the wet particles flowing down through the wetting zone in order to dry them. At least a portion of the particles flowing down is then entrained with the rising drying gas and conveyed back upwards to the wetting zone whereby the unreacted reagents or absorbents are activated again and the particle density in the reactor remains high.
At least a portion of the particles, agglomerating in the wetting zone or in the filter, is conveyed through the wetting zone to the lower portion of the reactor, whereas separate particles are re-entrained by the gas flow and they are conveyed back to the upper portion of the reactor already from the wetting zone. Larger particle lumps and wet, heavy particles do not dry until in the lower portion of the reactor, where they are dried by the drying gas or by some other mixing effect.
With the method described in patent application 900915 it is possible to lower the mean temperature of the gases in the wetting reactor to about 0.degree.-20.degree. C., preferably to 0.degree.-10.degree. C., from the dew point, i.e. practically to the dew point and still avoid the disadvantages caused by too wet particles in the upper or lower portion of the reactor. The particles wetted in the wetting zone and flowing downwards are dried by the drying gas in the drying zone and do not cause any trouble in the lower portion. The differences in temperature and moisture content are because of the recycling--very small in different locations of the reactor cross-section, thereby avoiding local harmful effects caused by wet particles or water drops.
Wet particles tend, however, to stick to moist surfaces, especially in the wetting zone. The water or water vapor sprayed from the nozzles creates good conditions for the particles to stick. The cold water flowing in the nozzle means effects condensation on the outer surfaces of the nozzle means and thereby also moistening thereof. The eddy current forming round the nozzle means naturally contributes to the sedimentation of particle layers on the outer surfaces. Rather thick layers can form on the nozzle means. The layers cause difficulties in spraying water or water vapor into the gas flow, so they must be removed. The layers can even direct a water spray in the wrong direction or completely clog some of the nozzles. In addition to this, the particles thus accumulated upon the nozzle means can loosen and drop in rather large lumps and, when falling down, disturb the operation in the lower part of the wetting reactor.
The Finnish patent publication 78777 suggests arranging the nozzles in a casing to protect the nozzles from mechanical damage. The particle layers forming gradually on the casing will, nevertheless, disturb the operation and finally clog the nozzles if special measures are not taken.
An object of the present invention is to provide an apparatus for and a method of accomplishing improved purification of gases.
A special object of the present invention is to provide for a method of and an apparatus for enabling the activating or wetting reactor to function so as to minimize the above-mentioned disadvantages due to the layering of particles.
Thus, a special object of the present invention is to provide for an activating or wetting reactor, in which the fouling and clogging of the nozzle means has been minimized.
In order to accomplish the above-mentioned objects, a nozzle apparatus according to the invention is characterized in that the nozzle body belonging to the nozzle apparatus is at least partly surrounded by a flexible and/or porous casing. A conduit has been arranged for introducing gaseous medium into the casing, whereby
the flexible casing is caused to change its form and/or
the medium flows out of the casing through the porous walls. The change of form or the medium flowing through the casing walls removes the solids accumulated on the casing, if any.
The method according to the invention is characterized in that gaseous medium is introduced into the casing surrounding the nozzle apparatus in order to cause the casing to change its form and/or to create a gas film on the casing, the change of form removing the layer, if any, of solids accumulated on the casing or the film preventing the forming of a layer of solids on the casing.
The nozzle body comprises a conduit for introducing the wetting medium to the nozzles, the nozzle body being formed of e.g. a straight or curved tube or an elongated hollow beam, arranged to span the activating reactor horizontally, or if desired, in an inclined position, from reactor wall to reactor wall. The nozzle body can be supported straight by the reactor walls or it can be fastened to the walls with separate support members. Either one end or both ends of the nozzle body is/are provided with inlet conduits for introducing the wetting medium, such as water, water vapor etc. therein. The nozzle body can be fitted, e.g., with evenly spaced nozzles for spraying the wetting medium into the gas flow. The nozzles can be directed as desired. The spray of wetting medium can be directed downward, it can be inclined to the side, or it can be directed upward. The medium can be sprayed concurrently with the gas flow or directly or obliquely against the gas flow. The nozzles can be pressure atomizing or pneumatically atomizing.
The nozzle body does not have to be tube- or beam-shaped. It can, for example, be formed of a circular channel. The circular channel is especially applicable in a cylindrical reactor, wherein it is advantageously arranged horizontally.
The nozzle body is, according to the invention, surrounded by a casing, preventing the particles from accumulating on the actual nozzle body. The casing is either flexible or porous so as to enable the particles accumulated thereon to be removed by raising the pressure in the casing.
The casing can be formed of a flexible, gas-proof hose, such as a rubber-based fabric-covered hose, a rubbered canvas-reinforced hose or other flexible hose able to withstand chemically and mechanically the warm and humid conditions of the activating reactor. In some cases an ordinary rubber or plastic hose can be utilized.
The casing is preferably threaded over the nozzle body so that it covers substantially the whole of the body. The casing is gas-tightly sealed with e.g. hose clamps to the end plates arranged at the ends of the nozzle body, thus closing the ends of the casing. The casing is provided with openings for the nozzles so that the tips of the nozzles stick out from the casing. The openings for the nozzles are gas-tightly sealed around the nozzles.
In addition to this, the casing is provided with a conduit for introducing the gaseous medium into the casing. Preferably the medium is introduced in the casing in pulses inflating the walls of the flexible casing as the pressure increases in the casing and thereby removing the particles accumulated on the casing. The pressure in the casing can be raised to e.g. 0.1-6 bar, and in between the pulses the pressure is allowed to decrease back to the pressure prevailing in the reactor or other surroundings. Preferably, the medium causing the pressure is air, but other gases or steam can also be utilized. The pulses of compressed air are introduced into the casing at, e.g., intervals of 0.5-60 minutes, according to present need for cleaning. This solution for removing the layers of particles can simply and inexpensively be applied to numerous different reactors.
According to another embodiment of the invention the nozzle body can be surrounded by a casing made of filter material, e.g. bag filter material. Thus the casing is both flexible and porous and the cleaning is effected by both the change of form of the casing walls and the gas flowing through the walls.
If desired, the nozzle body can be surrounded by a stiffer casing, made of e.g. metal sinter, which is very durable in even corrosive conditions. The casing to be arranged on the nozzle body can be made of e.g. sintered filter tube. The accumulation of particles on a metal sinter tube can be prevented with gas sprays flowing through the sinter material. It is specially useful to use metal sinter as material for the parts prone to gather particle layers and to direct the gas sprays through the sintered material to where they are needed to remove the particles from the tube walls. The gas can be introduced into the casing as impulses or as continuous flow.
The nozzle means according to the invention minimizes or eliminates the fouling and clogging of the nozzles by preventing the accumulation of particles on the nozzle means. Thus the operation of the nozzle means and an even spraying of the wetting medium in the gas flow can be ensured. The nozzle apparatus according to the invention prevents large particle lumps from forming on the nozzle apparatus. Thus, also the disadvantages, caused by particle layers to the operation of the activating reactor, and the disadvantages, caused by large lumps to the particle circulation and to the apparatuses, mixers and conveyers possibly located at the bottom of the reactor, can be avoided. Thus a nozzle apparatus according to the invention improves and stabilizes the whole operation of the activating reactor and also provides for wetting the gas to be purified to very near the dew point, even to the dew point in the wetting zone, as large, disadvantageous lumps are not formed. In this way, the whole of the gas purifying stage is improved and process gases are exhausted cleaner than before.
When compared to prior art techniques, the absorption of sulphur is improved with a reduced consumption of lime.
The casing arranged to surround the nozzle body also protects the nozzles from mechanical damage and wear. As the nozzles are directed downward, they are protected from particles and possible particle lumps falling from the filter in the upper portion of the activating reactor.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.